1. Field of the Invention
The present invention relates to changing of digital rights management (DRM), and more particularly, to a method of transmitting content processed by a DRM directly to a device that uses another DRM, and a method of reproducing the content.
2. Description of the Related Art
Digital rights management (DRM) indicates a method of managing digital contents so that only an authorized user can use them. In general, a DRM system includes a content server that provides content; a license server that gives a user a right to use the content, changes the right, or removes the right from the user; and a user device that receives the content from the content server, receives the right for the content from a license server, and uses the content.
A DRM server application program is installed in the license server, and a DRM client application program is installed in the user device. Various DRM programs have been developed and used, such as SmartRight developed by Thomson, Open Conditional Content Access Management (OCCAM) suggested by Sisco, an xCP Cluster Protocol developed by IBM, and Digital Transmission Content Protection (DTCP) developed by Digital Transmission License Administrator (DTLA).
FIG. 1 is a schematic block diagram of a conventional DRM system. Referring to FIG. 1, a device 1 sends a content server 2 a message 4 requesting content that a user desires to receive.
The content server 2 transmits content 5 processed according to DRM A to the device 1. That the content 5 is processed according to the DRM A is understood that the content 5 is encrypted using an encryption key and an encryption format defined in the DRM A, and that use control information regarding the content 5, which specifies usage rules and a number of times and a period of time that the content 5 can be reproduced, for example, is recorded in a format defined by the DRM A.
Next, the device 1 sends the license server 3 a message 6 requesting a license for the content 5. A license is a legal document giving official permission to use predetermined content. The license includes a decryption key for decrypting the content and the use control information. If the content is processed according to the DRM A, the device 1 must acquire a license for the content defined in the DRM A, in order to use the content.
The license server 3 determines whether the user is an authorized user, and generates a license 7 for the content 5 and transmits the license 7 to the device 1 when the user is an authorized user. The license 7 includes a decryption key and use control information specified by the DRM A.
Referring to FIG. 1, the license server 3 may be combined with the content server 2 to form a DRM server 8.
The device 1 reproduces the content 5 using the license 7. Specifically, a DRM client program installed in the device 1 extracts the decryption key for decrypting the content 5 and the use control information from the license 7, and checks a number of times and a period of time that the content 5 can be reproduced, which is described in the extracted use control information, to determine whether use of the content 5 violates rules defined in the use control information. If the use of the content 5 does not violate the rules, the content 5 is decrypted using the decryption key and the decrypted content is reproduced.
Since the data format of a license varies according to the type of a DRM system used, a DRM system must be changed to allow data to be exchanged between devices that use different DRM systems.
FIG. 2 is a diagram illustrating a conventional method of changing DRMs. Conversion of DRM A into DRM B allows a device Dev_B that uses the DRM B to receive and reproduce content Cont_A processed according to the DRM A.
In FIG. 2, a device Dev_A, which is currently being used, uses the DRM A, and the device Dev_B uses the DRM B. The content Cont_A is obtained by processing content Cont using the DRM A, and stored in the device Dev_A. The method shown in FIG. 2 will now be described in detail for a case where the device Dev_B desires to use the content Cont using the DRM B and the device Dev_A provides the content Cont to the device Dev_B. Throughout this disclosure, content Cont_A indicates content obtained by processing the content Cont according to the DRM A, and content Cont_B indicates contents obtained by processing the content Cont according to the DRM B.
Referring to FIG. 2, the device Dev_A sends a DRM conversion server S-conv a message indicating that the device Dev_B has used the DRM B and requires the content, together with the content Cont_A and a license Lic_A (S202).
Next, the DRM conversion server S_conv converts the content Cont_A into the content Cont_B using a server S_A (S204 and S206).
Next, the DRM conversion server S-conv requests and receives a license Lic_B required to use the content Cont_B from a server S_B corresponding to the DRM B (S208 and S210).
Thereafter, the DRM conversion server S_conv transmits the content Cont_B and the license Lic_B to the device Dev_B (S212). In this case, the content Cont_B and the license Lic_B may be transmitted to the device Dev_B via the device Dev_A or transmitted directly to the device Dev_B.
However, in the conventional method, the content Cont_A is transmitted from the device Dev_A to the DRM conversion server S_conv and the content Cont_B is transmitted from the DRM conversion server S_conv to the device Dev_B, thereby increasing load on a transmission line and the processing load on the DRM conversion server S_conv. In general, since a DRM conversion server is connected to lots of devices, this problem becomes worse.